Mr Joe Biden defeated President Donald Trump in Arizona, the Associated Press projected, becoming the first Democratic presidential candidate to win the state since 1996.
Mr Biden benefited from a more diverse electorate in Arizona, which Mr Trump won by 3.5 percentage points in 2016.
The state represents 11 of the 270 Electoral College votes needed to win the White House.
Mr Bill Clinton was the last Democrat to win Arizona, claiming it for his re-election.